<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <select name="" id="abc" onchange="select()">
        <option value="">请选择网站</option>
        <option value="http://www.baidu.com">百度</option>
        <option value="http://www.wanyi163.com">网易</option>
        <option value="http://www.taobao.com">淘宝</option>
    </select>
</body>
<script>
    function select(){
        var a = document.getElementById('abc').value;
        location.href = a
    }
</script>
</html>